A bodice ripper is a historical romance novel with naughty bits. The plot is centered on a man and a woman and the development of their relationship. Subplots often include a of mystery or threat that the budding couple must overcome. This type of subplot also allows for the hero to overcome his previously entrenched resistance to the matrimonial state and recognize his previously hidden romantic feelings when his love interest is kidnapped/shot at/near death or otherwise threatened.
Heroes in these books are usually tall, dark and handsome (though one will find the odd "golden god") and are described as rakes and rogues, experienced men who really know how to please their lady.
The women are a bit more varied, likely because these books are written for women. So although the heroine is always beautiful in the eyes of the hero, she may not fit into the conventional mold of beauty for the historical period. Most, sadly, are virgins. (One writer who frequently bucks the virgin trend, and has some of the most independent and interesting female characters, is Susan Johnson.)
One of the defining features of a bodice ripper is the description of sex. The heroes have "members" or "manhoods" of, um... heroic dimension! The heroines are endowed with heaving bodices and lush, flowery centers of their womanhood. (Example: his strong fingers parted her dewy petals and stroked that throbbing part of her, the center of her desire.) Oh yes, a lot of things tend to throb. While seduction is a clear theme, and the hero occasionally has to overcome the heroine's good sense (after all, sleeping with a rogue just after your first waltz hardly seems sensible), pseudo-rape is not necessary, and is less common in more recently written romances. For whatever reason (maybe fear of female sexuality?) the characters in the older books almost never got kinky without a bit of force.
Despite the occasionally over-florid descriptions of sexual intercourse, and the predictability of plot, many bodice rippers are well researched and well written.